Jesus, and his Christianity business has been around about 2000 years.
Allah has been around about 1500 years. they're both pretty young when it comes to discussing worldwide religion.
Jesus has been around a whole lot longer than that. After all, He's God. The Creator.
The key to understanding this statement is to know that Christianity is the only religion that is rooted in history. If you look at the first two chapters of Genesis, you see man in paradise as it was God's original purpose. If you read the last two chapters of the Revelation of John, you see man restored to paradise. Everything in between is what Christians call "salvation history," starting with the fall of man and ending with the redemption of man.
So, it's not accurate to say Christianity is only 2000 years old. It is, literally, older than dirt. When God became Incarnate, it wasn't a matter of saying "Hey, here's a new religion." It was the fulfillment of an old religion, not the establishment of a new religion.
Unfortunately, too many Christian believers don't actually understand the roots of their own faith. So why would that understanding be expected of non-believers? Hence, the way these threads almost always go.
Steve.